In recent years, a new type of gasoline combustion has been demonstrated. In particular, a pre-mixed air-fuel mixture is compressed in a combustion chamber such that the mixture combusts without using a spark plug to initiate the combustion. This type of combustion, referred as homogeneous charge compression ignition (HCCI) combustion, has been developed to improve fuel economy and emissions of gasoline engines. HCCI may also be referred to as compressed self ignition.
To perform this HCCI operation, a technology is known for keeping combusted gas in a combustion chamber of the engine as an internal exhaust gas recirculation (EGR) during a predetermined engine state, such as a partial-load state. This technology accomplishes this functionality by controlling exhaust valve closing timing and intake valve opening timing to provide a negative overlap period wherein both an intake and an exhaust valve are closed.
One example of a method of operating a gasoline engine using HCCI combustion is described by Japanese Unexamined Patent Application Publication No. 2006-22664. This reference describes increasing the concentration of oxygen supplied into a combustion chamber during a compressed self ignition operation when a desired engine condition is within a predetermined higher speed region or a predetermined higher load region to decrease engine output.
Another example of a method of operating a gasoline engine using HCCI combustion is described by U.S. Pat. No. 7,156,070. This patent describes a technology of injecting at least part of the fuel in a cylinder cycle as a pilot fuel in the negative overlap period to form radials, intermediates or combustion products that can control self ignition timing.
In particular, the self ignition timing is controlled by adjusting a pilot fuel amount in response to an engine knock signal and a combustion stability signal from engine operation sensors such as an engine knocking sensor, an in-cylinder pressure sensor, or an acceleration sensor.
Also, as a technology of providing the negative overlap period, the valve operating mechanisms described in Japanese Unexamined Patent Application Publication No. 2006-336494 and Japanese Unexamined Patent Application Publication No. 2006-348774 are well-known.
It is desirable to enlarge a partial load region where the compressed self ignition operation is performed to higher load regions to the extent possible in order to improve exhaust characteristics and fuel efficiency, and to decrease pumping losses by performing compressed self ignition operation.
However, the inventors herein have recognized that in relatively high load regions, providing a negative overlap period and increasing concentration of oxygen supplied into a combustion chamber according to an internal EGR amount as described above with respect to Japanese Unexamined Patent Application Publication No. 2006-22664, can make it easier to generate an unintended surface ignition within the cylinder, resulting in undesirable early ignition.
Meanwhile, as described above, another related art of U.S. Pat. No. 7,156,070 controls compressed self ignition timing by adjusting an amount of a pilot fuel injected into a combustion chamber in the negative overlap period in response to an engine knock signal and a combustion stability signal to improve engine combustion stability.
However, the inventors herein have recognized that undesirable early ignition can result in a decrease of combustion stability in this related art, especially at relatively high engine load and/or high engine speed, because self ignition of fuel occurs all at once. Thus, increase of in-cylinder temperature and pressure may occur within a relatively short period.
One aspect of the present description includes a method of controlling an internal combustion engine, the method comprising: closing an exhaust valve of a combustion chamber of said engine during a cylinder cycle prior to opening an intake valve of said combustion chamber; when a desired engine torque is a predetermined torque or greater, supplying a first pilot fuel into said combustion chamber after said exhaust valve closing during the cylinder cycle so that said first pilot fuel combusts before said intake valve opening during the cylinder cycle, and supplying a first main fuel into said combustion chamber after the combustion of said first preliminary fuel during the cylinder cycle so that said first main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing; and when a desired engine torque is less than said predetermined torque, supplying a second main fuel into said combustion chamber so that said second main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing while no combustion occurs between said exhaust valve closing and said intake valve opening during the cylinder cycle.
This method overcomes at least some of the disadvantages of above references.
When a desired engine torque is a predetermined torque or greater, by supplying a first pilot fuel into a combustion chamber after an exhaust valve closing or in negative overlap period so that a first pilot fuel is self ignited preliminarily, and supplying a first main fuel into a combustion chamber after the combustion of the first preliminary fuel so that the first main fuel is self ignited after an intake valve closes and combusts after the compression top dead center is passed, increase of in-cylinder temperature and pressure is divided into two stages, which can substantially reduce a possibility of undesirable early ignition of main fuel supplied after the exhaust top dead center and substantially improve combustion stability at relatively high engine loads.
Accordingly, such an approach can expand an operation range where HCCI operation can be performed to relatively higher engine loads.
In an example embodiment, a first pilot fuel is self ignited between said exhaust valve closing and said intake valve opening.
In one example embodiment, a first main fuel and second main fuel increase as the desired engine torque increases, and the first main fuel is less than the second main fuel when the desired engine torque increases across a predetermined torque.
This can be used to determine an amount of pilot fuel that will result in compressed self ignition in the negative overlap period, and to decrease fuel supplied after the exhaust top dead center, thereby inhibiting undesirable early ignition in a cylinder.
In one example embodiment, a first pilot fuel increases as the desired engine torque increases.
In another example embodiment, a period between an exhaust valve closing and an intake valve opening or a negative overlap period during the cylinder cycle increases as the desired engine torque increases when the desired engine torque is greater than a predetermined torque.
This can promote the performance of pilot fuel compressed self ignition in the negative overlap period and improve combustion stability in this stage. Further, this can decrease fuel supplied after the exhaust top dead center, which can inhibit undesirable early ignition in a cylinder.
In one example embodiment, an intake valve opening retards as the desired engine torque increases when the desired engine torque is greater than said predetermined torque.
In another example embodiment, a period between an exhaust valve closing and an intake valve opening during the cylinder cycle decreases as the desired engine torque increases when the desired engine torque is less than a predetermined torque.
This results in decreasing internal EGR and increasing an effective compression ratio, which can expand an operation range where HCCI operation can be performed to relatively higher engine loads.
In another example embodiment, a period between said exhaust valve closing and said intake valve opening during the cylinder cycle increases when the desired engine torque increases across said predetermined torque.
A second aspect of the present description includes a method of controlling an internal combustion engine, the method comprising: closing an exhaust valve of a combustion chamber of said engine during a cylinder cycle prior to opening an exhaust valve of said combustion chamber; when an engine speed is a predetermined speed or greater, supplying a first pilot fuel into said combustion chamber after said exhaust valve closing during the cylinder cycle so that said first pilot fuel combusts before said intake valve opening during the cylinder cycle, and supplying a first main fuel into said combustion chamber after the combustion of said first pilot fuel during the cylinder cycle so that said first main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing; and when an engine speed is less than said predetermined speed, supplying a second main fuel into said combustion chamber so that said second main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing while no combustion occurs between said exhaust valve closing and said intake valve opening during the cylinder cycle.
This method also overcomes at least some of the disadvantages of the references described above.
When a desired engine speed is a predetermined speed or greater, by supplying a first pilot fuel into a combustion chamber after an exhaust valve closing or in negative overlap period so that a first pilot fuel is self ignited preliminarily, and supplying a first main fuel into a combustion chamber after the combustion of the first preliminary fuel so that the first main fuel is self ignited after an intake valve closes or combusts after the compression top dead center is passed, increase of in-cylinder temperature and pressure is divided into two stages, which can substantially reduce a possibility of undesirable early ignition of main fuel supplied after the exhaust top dead center and substantially improve combustion stability at relatively high engine loads.
Accordingly, such an approach can expand an operation range where HCCI operation can be performed to relatively higher engine speeds.
A third aspect of the present description includes a method controlling an internal combustion engine, the method comprising: closing an exhaust valve of a combustion chamber of said engine during a cylinder cycle prior to opening an exhaust valve of said combustion chamber; when an engine torque is a predetermined torque or greater or an engine speed is a predetermined speed or greater, supplying a first pilot fuel into said combustion chamber after said exhaust valve closing during the cylinder cycle so that said first pilot fuel combusts before said intake valve opening during the cylinder cycle, and supplying a first main fuel into said combustion chamber after the combustion of said first pilot fuel during the cylinder cycle so that said first main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing; and when an engine torque is less than said predetermined torque and an engine speed is less than said predetermined speed, supplying a second main fuel into said combustion chamber so that said second main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing while no combustion occurs between said exhaust valve closing and said intake valve opening during the cylinder cycle.
This method also overcomes at least some of the disadvantages of the references described above.
When a desired engine torque is a predetermined torque or greater, by supplying a first pilot fuel into a combustion chamber after an exhaust valve closing or in negative overlap period so that a first pilot fuel is self ignited preliminarily, and supplying a first main fuel into a combustion chamber after the combustion of the first preliminary fuel so that the first main fuel is self ignited after an intake valve closes or combusts after the compression top dead of center is passed, increase of in-cylinder temperature and pressure is divided into two stages, which can substantially reduce a possibility of undesirable early ignition of main fuel supplied after the exhaust top dead center and substantially improve combustion stability at relatively high engine loads.
Accordingly, such an approach can expand an operation range where HCCI operation can be performed to relatively higher engine torques.
A fourth aspect of the present description includes a system comprising: an internal combustion engine having a combustion chamber which is defined by a reciprocating piston and a cylinder accommodating said piston, an intake valve capable of opening and closing communication between an air intake path of said internal combustion engine and said combustion chamber, and an exhaust valve capable of opening and closing communication between an exhaust path of said internal combustion engine and said combustion chamber; a fuel injector capable of supplying fuel into said combustion chamber; an intake valve actuator which actuates said intake valve; an exhaust valve actuator which actuates said exhaust valve; and a controller which is configured to control: said intake and exhaust valve actuators to close said exhaust valve prior to opening of said intake valve during a cylinder cycle; said fuel injector, when a desired engine torque is a predetermined torque or greater, to supply a first pilot fuel into said combustion chamber after said exhaust valve closing during the cylinder cycle so that said first pilot fuel combusts before said intake valve opening during the cylinder cycle, and to supply a first main fuel into said combustion chamber after the combustion of said first pilot fuel during the cylinder cycle so that said first main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing; and said fuel injector, when a desired engine torque is less than said predetermined torque, to supply a second main fuel into said combustion chamber so that said second main fuel is self ignited after said intake valve closes during the cylinder cycle and combusts after a top dead center after said intake valve closing while no combustion occurs between said exhaust valve closing and said intake valve opening during the cylinder cycle.
This system also overcomes at least some of the disadvantages of the above described references.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.